696 Injectors. is some 16 or 18 times that of water at the same pressure, and the jet of steam escaping from D is so suddenly cooled by the tank water through c, that it has not time to reduce its velocity to that due to it as water, and therefore succeeds in piercing the boiler water, carrying the tank water with it. An overflow takes place at F when first starting, which, however, ceases when cocks c and H have been mutually adjusted. A lifting injector must permit of regulation at the orifice D and ring orifice E, for the conditions of vacuum-forming and water-forcing are quite different, and the former must be first satisfied, after which the latter may be met without disruption of the water column, An ingenious method of automatic lifting injector is now in operation, where the throat E is split longitu- dinally, and one half hinged near the annulus, the' flap nozzle' thus formed also causing are-starting, should the fluid tend to disunite, Slightly altering the cone proportions, and giving the water a few feet of head, produces an injector workable by exhaust steam ; but, on account of the variation in pressure, the flap nozzle must be provided. Other Mountings for the boiler are: a blow-off cock near the firebox bottom, gauge cocks about 3 ins. above and below the water line, fire bars and bearers, furnace doors, rnud plugs, fusible plug in furnace crown to melt in case of overheating, and thus •cause the fire to be extinguished, clack box (j, Fig. 682), damper for regulating draught, a filling branch when no other hole is -convenient, and sometimes a scum cock. (SeeApp. •//.,/. 902.) Combustion.—Combustion or burning is rapid chemical combination accompanied by heat and sometimes light. If con- siderable noise be caused it is termed an explosion. During •combination, heat is produced equal to that required to separate the same elements. The separation of carbon and hydrogen, and their re-combination with oxygen, is what the engineer needs to understand, so \ve will consider the burning of a simple hydrocarbon like marsh gas, shewn by the formula : Marsh gas 4- Oxygen. = Carbon dioxide -t- Water (steam) CO, 2 BLO a case of complete combustion, for no single element remains.